Extreme insect fashion is electric

If you've ever been looking at a knot on a branch only to see to it start walking, you may have been looking at a treehopper. Treehoppers, a group of insects related to cicadas and leafhoppers, are known amongst biologists for their strange body shape. The little insects have a wide range of spikes, spines and curves, which has long made biologists question their function. But a recent study has finally shed light on this riddle, revealing that these freakish morphologies may help treehoppers sense the invisible world of electricity. Sam England and colleagues at the University of Bristol, UK, have uncovered a hidden sense in treehoppers, one that doesn't rely on sight or smell, but on the electric fields surrounding them. By combining field measurements, behavioral tests and computational simulations, researchers demonstrated not only that treehoppers can detect electrical charges but also that their complex head shapes may actually act as a sort of antenna for these charges.Past research has shown that other insects such as bumblebees are sensitive to electric fields, so the team investigated whether the same might be true of the treehoppers. First, the team took a look at the electric fields produced by the treehoppers and some of their common predators, such as wasps. While based at the Estacion Biologica Le Selva, Costa Rica, England and colleagues recorded the electric fields of almost 500 insects, including 11 different treehopper species, two species of wasp and two species of stingless bee. To the researcher's surprise, the treehoppers showed consistent differences in their electric profiles. These differences extended to other insects in the treehopper's surroundings. For example, stingless bees, which live among the treehoppers, had a different signature to the wasps, providing an electric code for friend or foe.With these field-based measurements in hand, the researchers employed the help of computer simulations to understand what the relationship between the electric signals in the environment was to the varying shape of treehopper species' heads. The results of the simulations showed that varying head shape can amplify the strength of signals up to fivefold. To further confirm the role of the unique head shapes, the researchers exposed various treehoppers to electrical fields, observing that they retreated when an electrical field was present. This served as definitive evidence that the little insects were using an additional sense.To uncover how the insects might be sensing the electrical fields, the researchers used scanning electron microscopy to study the insects in incredible detail. They found a number of stiff, hair-like structures called setae that can be used to sense electrical fields. The number and location of these hairs changed with different treehopper head types, increasing the link between head shape and sensing. With these three components of their study, the researchers were able to conclude that the head shapes of treehoppers allow them to sense electric fields useful for avoiding predators.This discovery is important not only for those who study treehoppers but also for animal biology more generally. Showing that another group of insects have this additional sensing ability opens up new fields of study in animal perception. Additionally, biologists have long been interested in the connection between form, how an animal looks, and function, what that animal does. This is yet another example of form and function being coupled together. It turns out that being striking isn't the only reason for these little insects' headwear.